Production of terephthalic acid and esters



United States Patent 2,782,224 I PRDDUCTION F TE'REPHTHALIC ACID AND ESTERS Peter Smith, Norton-on-Tees, England, assignor to Imperial Chemical Industries Limited, a corporation of Great Britain No Drawing. Application April 23, 1954, Serial No. 425,303

Claims priority, application Great Britain May 1, 1953 8 Claims. (Cl. 260-475) This invention relates to the production of cyclohexa- 1,4-dicarboxylic acid and acid esters and di-esters thereof. According to the present invention, there is provided a process for the production of cyclohexa-1,4-diene-l,4- dicarboxylic acid or acid esters or di-esters thereof which comprises the step of heating at a temperature of at least 150 C. cyclohexane-2,5-diol-1,4-dicarboxylic acid or acid esters or di-esters thereof, containing, in solution, at least 0.01% by Weight, based on the amount of acid or acid ester or di-ester, of a di-alkali metal salt of cyclohexane-2,S-diol-1,4-dicarboxylic acid.

The preferred di-alkali metal salt of cyclohexane-2,5- diol-l,4-dicarboxylic acid for use in the present invention is di-sodium cyclohexane-2,5-di ol-1,4-dicarboxylate. The amount of dialkali metal salt must exceed 0.01% of the weight of acid or acid ester or di-ester employed; an amount of 1% to 5% is preferable, but larger amounts may be employed.

By the process of the present invention, cyclohexane- 2,5-diol-1,4-dicarboxylic acid may be converted to cyclohexa-l,4-diene-1,4-dicarboxylic acid. In a preferred application of the process of the present invention, diinethyl cyclohexane-2,5-diol-1,4-dicarboxylate is converted to dimethyl cyclohexa-l,4-diene-1,4-dicarboxylate.

The present invention provides an important step in the conversion of a di-alkyl sucoinate to terephthalic acid or an acid ester or di-ester thereof. Thus, a di-alkyl succinate may be condensed by means of a sodium or sodium alkoxide catalyst to give a disodium derivativeof cyclohexa-1,4-diene-2,5-diol-1,4-dicarboxylate. This may be hydrogenated and hydrolyzed to give cyclohexane-2,5- diol-1,4-dicarboxylic acid or an acid ester or di-ester thereof which may then be converted by the process of the present invention to cyclohexa-l,4-diene-1,4-dicarboxylic acid or acid esters or di-esters thereof which are useful as, for example, starting materials for the production of terephthalic acid or acid esters or di-esters thereof, by the process described in copending application Serial No. 419,576, filed March 29, 1954.

The starting materials for use in the present process can exist as geometrical isomers. Some of these isomers tend to give rise to lactones. For example, it is believed that when using dirnethyl cyclohexane-2,5-diol-1,4-dicarboxylate and when the hydroxyl group in the 5-position is cis to the carbornethoxy group in the l-pos-ition, the lactone 4-carbomethoxy 2 hydroxy-6-oxabicycl'o-[3 :2: 1]- 0cta-7-one is formed as a by-product. This lactone formation is diminished by working in a solvent, such as water or an aliphatic alcohol containing at most four carbon atoms. It is particularly advantageous to use as solvent the alcohol corresponding to the ester employed; for example, when using dimethyl cyc1ohexane-2,5-dioll,4-dicarboxylate as the starting material, it is advantageous to use methanol as the solvent.

When using a di-alkyl cyclohexane-2,5-diol-1,4-dicarboxylate as the starting material, and operating either in the absence of a solvent or in the presence of water,

the presence of Water or an alcohol as solvent, there is acid or acid ester or di-ester thereof, instead diol- 1 ,4-dicarb'oxylate and,

alkyl hydrogen cyclohexa-l,4-diene-1,4-dicarboxylate and cyclohexa-1,4-diene-1,4-dicarboxylic acid are presentin the reaction product. Under similar conditions, using an alkyl hydrogen cyclohexane-Z,5-diol-1,4-dicarboxylate as the starting material, the product contains cyclohexa-1,4-diene-1,4-dicarboxylic acid.

If an alcohol solvent is used in conjunction with a (ii-ester, and the said alcohol does not correspond to the di-ester, then the product will tend to contain mixed esters of cyclohexa-1,4-diene-1,4-dicarboxylic acid.

The process of the present invention is carried out at an operating temperature of at least 150 C.; the operating temperature should be lower than the critical temperature of the solvent, if one is employed. In particular, an operating temperature of 'the order of 200 C. is

satisfactory. It is, of course, necessary to operate at a pressure sufiicient to maintain the solvent in the liquid phase. 1

In a modified mode of operation, the alkali metal salt is provided by adding the requisite amount of alkali metal hydroxide to the cyclohexane-2,5-diol-1,4-dicarboxylic of adding a dialkali metal cyclohexane 2,5 diol-1,4-dicarboxylate. When operating in this manner, the alkali metal hydroxide reacts with cycl'ohexane-2,5-diol-1,4-dicarboxylic acid or acid ester or di-ester thereof to give the requisite dialkali metal cyclohexane-2,5-diol-l,4-dicarboxylate.

The present process possesses distinct advantages over those methods previously disclosed for the dehydration of cyclohexane-2,5-diol-1,4-dicarboxylic acid or acid esters or di-esters thereof. It cal than those methods which involve the use of an acid dehydrating agent such as phosphoric acid or sulphuric acid; furthermore, when using a di-alkyl cyclohexane-2,5- for example, sulphuric acid, there is a considerably greater tendency than in the process of the present invention for the ester groups to be hydrolysed to the free acid. The present process is more rapid than thermal dehydration, and, when operating in diminished tendency of lactone formation, it there is a likelihood of this occurring. In co-pending application Serial No. 420,750, filed April 2, 1954, now U. S. Patent 2,734,077 it is disclosed that the dehydration can be effected by using a major amount, for example, at least two moles per mole of acid or acid ester or di-ester, of alkali metal hydroxide or alkoxide; however, irrespective of the use of an acid or acid ester or di-ester as the starting material, the major dehydrated product is the dialkali metal salt of cyclohexa-1,4-diene-1,4-dicarboxylic acid. Thus, to prepare a di-alkyl cyclohexa-1,4-diene- 1,4-dicarboxylate from a dialkyl-2,5-diol-1,4-dicarboxylate it is necessary to hydrolyse the di-alkali metal cyclohexa-l,4-diene-1,4-dicarboxylate, and to esterify the free acid. This process is wasteful in time and materials compared with that disclosed in the present application.

Example 1 18.4 grams of dimethyl cyclohexane2,5-dio1-1,4-dicarboxylate and 0.05 gram of the corresponding disodium derivative were heated in a stainless steel autoclave with rnls. methanol at 200 C. for 24 hours. On cooling to 20 C., 2.7 grams of dimethyl cyclohexa-1,4-diene-1,4- dicarboxylate separated out. The methanol solution was removed, concentrated and steam distilled when a further 1.75 grams of dimethyl cyclohexa-1,4-diene-1,4-dicarboxylate were obtained. The steam distillate on continuous extraction with ether gave a further 4.1 grams of dimethyl cyclohexa-1,4-diene-1,4-dicarboxylate. Thus the total is more convenient and economi methoxy-2-hydroxy-6mxa -bicyclo [322:1] octa-7-one' which could be recycled to a subsequent dehydration step.

Example 2 Example 1 was repeated using 0.5 gram of the disodium derivative instead of 0.05 gram. From the product the methanol was removed and 11.7 grams of dimethyl cyclohexa-1,4-diene-1,4-dicarboxylate were isolated. This corresponds to a yield of 75%. 4.3 grams of 4-carbornethoxy-Z-hydroxy-6-oxa-bicyclo [312:1] octa-7-one were isolated, corresponding to a yield of this lactonc of 23%. This lactone could be recycled-to subsequent dehydration steps. Thus it will be seen that in this example there is almost no loss of starting material by conversion to undesired products.

Example 3 105 grams of di-ethyl cyc1ohexane-2,5-diol-l,4-dicarboxylate in 500 mls. of ethanol together with 2 grams of sodium hydroxidewere heated for 36 hours at 220 C. in a stainless steel autoclave. In the course of this reaction a small amount of di-ethyl ester would be converted to disodium cycloheXane-2,5-diol-1,4diearboxylate and this would catalyse the dehydration reaction. From the reaction product, ethanol was removed by distillation and 75 grams of di-ethyl cyclohexa-1,4-diene-l,4-dicarhoxylate were obtained. This di-ester contained no hydroxyl groups; in other words the dehydration reaction was complete. The di-ester boiled at a temperature of 106 C., at a pressure of 0.6 mm. of mercury, and had a purity of 92%. The pass yield of di-ethyl cyclohexa-l,4-diene-1,4- 1 i dicarboxylate in this example was 76%.

I claim:

1. In a process for the preparation of organic compounds of the formula:

by dehydration and dehydrogenating a starting material comprising. a compound having the formula:

COOX

CH C CHOH HO 311: 112 cfi C EOOX wherein X is lower alkyl, respectively, the improvement which comprises conducting said dehydration by heating in the liquid phase said starting material at a temperature of at least 150 C. in a solution containing from 0.01% to 5% by weight of a di-alkali metal salt of cyclohexane- 2,5'diol-l ,4-dicarboxylic acid.

2. The process of claim 1 wherein said dehydration is carried out at 200 C.

3. The process of claim 1, wherein said salt is present :in solution in an amount from 1% to 5% by weight.

4. The process of claim 1, wherein said alkali metal is sodium.

5. The process of claim 4, wherein said X is a methyl group and said dehydration is carried out in the liquid phase in the presence of methanol.

6. The process of claim 4 wherein said X is an ethyl group and said dehydration is carried out in the liquid phase in the presence of ethanol.

7. The process of claim 1. wherein said di-alkali metal salt is formed in situ by heating said starting material with the requisite amount of alkali metal hydroxide.

8. The process of claim 1 wherein said starting material is heated in the presence of a hydroxylic solvent therefor selected from the group consisting of water and aliphatic alcohols containing at most, four carbon atoms.

References Cited in the file of this patent UNITED STATES PATENTS 784,411 Merling et al Mar. 7, 1905 784,412 Merling et a1. Mar. 7, 1905 2,734,077 Smith Feb. 7, 1956 FOREIGN PATENTS 738,922 Germany Sept. 21, 1943' OTHER REFERENCES Richter: Textbook of Organic Chemistry," pp. 75, 326, J. Wiley, 1952. 

1. IN A PROCESS FOR THE PREPARATION OF ORGANIC COMPOUNDS OF THE FORMULA: 